At the present time, there is considerable effort and interest in advanced miniaturization including the development of ultra small sensors, power sources, communication, navigation and propulsion systems.
In particular, micro heat engines (MHEs) driven by renewable energy sources have promising potential in numerous applications such as portable power generation, micro energy sources, waste heat recovery, fluid control, propulsion, cooling systems for electronics and people, and a variety of other applications. For example, one of the most beneficial applications of a micro heat engine would be to replace batteries as a means to power electronics. As is well known, with people's increasing dependence on portable electronics, batteries continue to be problematic in terms of their bulk, cost and the amount of power they are able to generate. Often, the batteries used in cell phones, laptop computers and digital cameras are not powerful enough to drive the many features now available in these devices for a reasonable length of time and as a result, there continues to be a need for alternate portable power supplies. Such systems could avoid the need of conventional batteries to re-charge (or be disposed of) by generating sufficient power (tens of watts of power) from solar energy or other heat supplies.
Other examples of applications for micro heat engines may include the following: Tiny machines being developed with microtechnology are many times smaller than the batteries available to power them with the result that there is little economic incentive to produce micromachines as there are currently no correspondingly small power sources to match.
In fluid control, micro-engines could be embedded within surfaces for boundary layer and fluid control where, for example, pulsating micro-pistons or micro-tabs enabled by micro-engines beneath a heated surface could be used to delay boundary layer separation and reduce wall friction.
Still further, micro heat engines could be used as energy sources for micro air vehicles and sensors as well as producing electricity from waste heat emitted from various sources such as the surface of a computer case.
In all of these examples, clean energy sources can be used to provide the input power to the micro heat engine, without generating greenhouse gases.
By way of comparison, to conventional macro-scale power generation systems, a conventional gas turbine generator with a 1 m diameter air intake area can generate power on the order of 100 MW. In theory, by scaling a conventional gas turbine system down to a millimeter size and maintaining a similar power per unit of airflow, tens of watts of could be produced from an MHE to meet the power requirements of many applications.
As a result, it is anticipated that an MHE could achieve close to the same level of specific power as a macro-scale engine, and possibly at an equivalent cost per unit output with future micro-machining advances. It is advantageous that within such a system, renewable energy sources could be better utilized.
However, it is also known that the energy and fluid transport processes influencing the design of any micro-components change appreciably from a macro to micro scale, i.e., friction, thermocapillary and other forces. In particular, the fabrication of micro turbomachinery for heat engines poses several challenges including the difficulty of creating detailed features at micron-scales, stresses on highly loaded parts and assembly and packaging. Also, current fabrication techniques cannot precisely duplicate the 3-D geometries encountered in macro-scale turbomachinery, so machine design becomes increasingly difficult when scaled-down versions of conventional devices cannot be constructed.
Still further, as the physical dimensions of microdevices decrease, surface and electromagnetic effects become increasingly significant relative to gravitational, inertial and viscous forces on the fluid. Electric charge patterns along the walls of a microchannel can be manipulated to control electromagnetic forces and fluid transport through a microdevice [1]. Voldman et al. [2] have developed a cantilever-plate flapper microdevice for controlling fluid motion through a microvalve. The device permits mixing and throttling capabilities, simultaneously. Scalable control of microfluidic transport has been implemented with microvalves fabricated in parallel arrays [3]. It was reported that effective control of flow rates up to 150 ml/min with a pressure differential of 10 kPa could by achieved within the microsystem.
An increasingly important microfluidic force affecting the fluid motion is surface tension. Kim [4] documents how surface tension forces can be effectively utilized and controlled in microfluidic transport processes. Thermocapillary pumping (TCP) is a non-mechanical, surface-tension driven pumping system for moving discrete liquid droplets in Microsystems. The small length scales of microdevices lend themselves well to flow control with thermocapillary pumping. A spatial temperature gradient within the microchannel generates a difference of surface tension and pressure across the droplet. The resulting pressure gradient causes the droplet to accelerate in the direction of lower pressure, similar to Poiseuille flow in a tube. In addition to Poiseuille flow characteristics, this article reports that additional re-circulation near the walls contributes to droplet acceleration. Unlike other applications involving dispersed droplet transport such as multiphase icing problems [5, 6], this article solves the full Navier-Stokes equations to predict internal motion within the droplet.
TCP has been applied to lab-on-a-chip technology. For example, blood and urine samples are typically sent to labs to be analyzed by various processes and results from the sample may talk several days. Additionally, errors such as mislabelling and lost samples may hamper with processing of data [7]. A lab-on-a-chip is a compact point-of-care clinical testing device, whereby all operations can be performed instantly on a small chip. Nanoliter samples of fluid can be transported within the chip to various processing stages using TCP. Since the fluid does not contact any physical pumping mechanism, there is no opportunity for a sample to become contaminated. Unlike conventional micro-pumps, another advantage of TCP is that it can move fluid in either direction, depending on the location of the surface heat source.
In addition to the lab-on-a-chip, another innovative TCP application involves optical fibres, when a small moving micro-droplet can re-direct a light beam by refraction or reflection to a different path [8]. Once the beam enters the fiber, it can be trapped by these internal reflections. Thermocapillary transport can be used as an optical “switch” to re-direct light from one fiber to another fiber. More specifically, the droplet in the microchannel would intersect the fiber-optic light beam. When the micro-droplet moves to the position of the intersecting light beam, the beam is reflected to a different fiber. Recent numerical studies involving FLOW-3D simulations [8] have utilized thermocapillary convection in a 14-micron channel (heated at the bottom boundary) for optical fiber applications.
DeBar and Liepmann [9] have manufactured and tested silicon and quartz thermocapillary pumps with a square cross-sectional area. Three heaters were used, with one heater to generate a fluid-vapor interface and two other heaters to control the spatial temperature gradient. The pumps were operated under a variety of conditions, in order to evaluate performance characteristics. These studies considered a square cross-sectional area, while other past studies have investigated other types of geometrical profiles, such as circular microtubes [10], surface micro-grooves [11], trapezoidal microchannels [12] and open rectangular microchannels [13]. In parallel channels, Glockner and Naterer [14] have shown that Reichhardt's Law can be effectively applied over multiple regions in the near-wall layer.
Sammarco and Burns [15] describe how TCP can be used to pump small volumes of fluid within a microfabricated flow channel. TCP velocities up to 20 mm/min were measured for toluene (ΔT≈26° C.), with temperature differences ranging from 10° C. (5 V) to 70° C. (18 V) for mineral oil. Experiments yielded TCP velocities that were comparable to theoretical expressions derived for the TCP velocity [15]. A subsequent paper by the same authors [16] describes a heat transfer analysis of TCP within the microchannel. The analysis investigates both fluid flow and energy transport to develop materials, designs and operational guidelines for effective pumping performance. For pumping velocities less than 0.1 cm/s, it was shown that a uniform interface temperature is possible, when pumping water across a fused silica substrate within a glass microchannel.
Predictions of TCP within a closed microchannel differ from an open microchannel, since the external pressure upstream and downstream of the droplet cannot be assumed equal. This pressure difference must be included in the analysis. Large droplet velocities reported in past TCP studies with open channels cannot be obtained in a closed microchannel, as small droplet displacements create an opposing pressure gradient in the gas phase with a similar magnitude as the thermocapillary pressure. Also, open-channel assumptions of steady-state Poiseuille flow are not fully applicable in closed microchannels, when cyclic heat input generates periodic acceleration and deceleration of the droplet. This discussion develops numerical and analytical formulations under these cyclic conditions. A detailed two-dimensional transient flow solution is developed to predict the detailed pressure and velocity changes in a new TCP application.
Other past work has included the work of researchers at Washington State University (D. Bahr, B. Richards, C. Richards) who built a micro heat engine for military applications. However, this system requires fuel for continuous operation.
Researchers at the University of Alberta (D. Kwok, L. Kostiuk) have also built a micro engine that generates electricity from microfluidic motion. This system requires pressurized water and a micro-turbopump. In another design, this group has also designed a microfluidic engine that uses water forced through a glass micro-channel to produce a streaming electric current by pushing positively charged ions, collected in the water at the boundary of the micro-channel, in the direction of the water current.
The literature has also revealed Electrokinetic microchannel battery by means of electrokinetic and microfluidic phenomena, published in 2003 in the Journal of Micromechanics and Microengineering by Yang et al.
A review of the patent literature has revealed US patent application No. 2002/0043895 which describes a piezo-electric micro-transducer. In one example, a working fluid operating as a saturated liquid-vapour mixture is heated and cooled by switches selectively coupled to a heat source for flexing the transducer. In other examples vibrations from a moving body, a pulsating pressurized fluid flow or combustion of fuel and an oxidizer mixture provide the flexing movement of the transducer.
As a result, there remains a need for systems that enables the operation of an MHE from any suitable latent heat input such as renewable solar energy or waste heat and that does not require micro machinery.